There are many efficient sorbents for oxygen at moderate temperatures. Commercial oxygen sorbent (BASF catalyst R-3-11) which is believed to comprise 5-6 metals deposited on a silica support has an upper temperature limit of 250.degree. C. and is degraded by accidental exposure to atmospheric air at room temperature. In Principle, transition metals such as copper and cobalt are capable of reducing trace oxygen contamination in inert gases to sub parts-per-billion (ppb) levels over a wide temperature range (100.degree.-800.degree. C.). The favorable thermodynamics for these two metals is readily apparent from a consideration of equilibrium partial pressures of oxygen over copper or cobalt as shown in the following table.
TABLE I: ______________________________________ EQUILIBRIUM OXYGEN PARTIAL PRESSURES RESULTING FROM FORMATION OF COPPER AND COBALT OXIDES Po.sub.2 based on Po.sub.2 based on Po.sub.2 based on Temperature CuO Cu.sub.2 O CoO (K) (atm) (atm) (atm) ______________________________________ 400 1.0 .times. 10.sup.-31 600 3.0 .times. 10.sup.-18 1.8 .times. 10.sup.-22 700 2.0 .times. 10.sup.-14 2.9 .times. 10.sup.-18 800 1.4 .times. 10.sup.-11 4.1 .times. 10.sup.-15 5.40 .times. 10.sup.-24 900 2.1 .times. 10.sup.-9 1.1 .times. 10.sup.-12 1.37 .times. 10.sup.-20 1000 9.4 .times. 10.sup.-.sup.11 7.19 .times. 10.sup.-18 1100 1.19 .times. 10.sup.-15 1200 8.47 .times. 10.sup.-14 1300 3.13 .times. 10.sup.-12 ______________________________________
These values were obtained from the literature values of free energies of species involved in the overall oxidation reaction and the relationship EQU .DELTA.G.degree.=-RT ln K.sub.p
where .DELTA.G.degree. is the free energy change for the reaction and K.sub.p is the equilibrium constant.
Even though the equilibrium oxygen levels are very low at high temperature (400.degree. C.) the oxidation kinetics are very slow. Oxidation kinetics of transition metals such as copper can be catalyzed by supporting the metals on zeolites. Apparently, the oxidation is catalyzed by acid sites on the zeolite and leads to high rates of copper-utilization (above 75%) at the upper end of the temperature range (500.degree. C.). Cobalt has significantly better oxygen equilibrium removal values. These supported catalysts are stable at room temperature requiring heating to an elevated activation temperature above 150.degree. C.
Furthermore, there are several processes for forming special materials that require oxygen removal to parts per trillion level at temperatures up to 1000.degree. C. For example, processing of some materials and alloys in micro-gravity conditions and of certain semiconductors require high temperature and trace oxygen removal. Cobalt exchanged zeolites are capable of efficient oxygen absorption at temperatures up to the structural stability limit of the zeolite support.
Preparation of transition metal exchanged zeolites is relatively straightforward. However, reduction of such materials is not always practical. The zeolites are only stable up to temperatures of from 600.degree. to 800.degree. C. After extensive treatment of the cobalt containing zeolite sorbents with hydrogen at temperatures from 200.degree. to 650.degree. C. for periods up to 24 hours, there was little change of color of the samples, indicating a substantial lack of reduction.
After exposure of cobalt exchanged zeolite L and 13X sorbents with 4% hydrogen--argon gas mixture at 600.degree. C. for 16 hours followed by contact with oxygen at 600.degree. C. only a small amount of oxygen uptake occurred consistent with lack of reduction. While 600.degree. C. is somewhat outside the stability limit of zeolite 13X, zeolite L is stable at temperatures up to about 800.degree. C. An attempt to reduce cobalt exchanged zeolite L even at 800.degree. C. with hydrogen was not successful.